The prior-art European Patent Application EP 1 762 897 A1 shows, in a lithographic system, a position-measuring device having a plurality of scales which are arranged end-to-end to extend the measurement range. Each of the plurality of scales has a reflective phase grating. The scales are disposed on a support (metrology frame) in a manner that enables continuous position measurement across the junction of two scales. The scanning unit is adapted to enable continuous position measurement across the junction. In European Patent Application EP 1 762 897 A1, the scales are referred to as “grid plates” and are adapted for position measurement in two directions that are orthogonal to each other.
It is known that the diffraction efficiency of a reflective phase grating may be affected by many parameters.
The relative diffraction efficiency is affected by the shape of the phase steps. Parameters for the shape are the profile of the phase steps, in particular the flank angle, and the outer contour of the reflective surfaces of the phase steps.
It is known from European Patent EP 0 849 567 B1 that the diffraction efficiency of a reflective phase grating can be optimized by suitably selecting the profile of the phase steps. By suitable selection of the flank angle, it is possible to optimize the relative diffraction efficiency.
European Patent Application EP 1106972 A1 shows that the outer contour of the marks of a phase grating of a scale may be rectangular, with the reflective rectangles being arranged two-dimensionally in a checkerboard pattern with the corners adjacent to each other. It is also generally pointed out that the marks may alternatively also be circular ring-shaped.